The use of non-rising stem gate valves to control the flow of high pressure liquid or gas is well known in the energy producing industry. It is also known to use a metal-to-metal backseat seal between the valve stem and the valve bonnet to seal the inner valve chamber from the exterior environment in order to allow repacking of the valve stem or replacement of the valve stem journal bearings. See for example U.S. Pat. No. 4,149,558 "Selective Backseat Valve", by McGee et al.
In the oil and gas producing industries, this valve has been further modified so as to automatically backseat under high temperature conditions, such as a fire, during which the resilient packing seal between the valve stem and the valve bonnet may be damaged and allow leakage. This modification, disclosed in U.S. Pat. No. 4,289,157 "Valve With Heat-Responsive Bearing Assembly Providing Backseat Arrangement", by McGee, includes a fusible washer-like annulus placed in the valve stem journal bearing assembly. Upon heating of the valve to an abnormally high temperature, the fusible annulus melts and runs out of a passageway to the exterior of the valve. The valve stem is now free to rise a limited extent due to the internal valve chamber pressure, thereby engaging the annular metal-to-metal sealing surfaces provided on the valve bonnet and the valve stem.
While this design is satisfactory, it does not meet the requirement that there be a positive metal-to-metal backseat that is not dependent upon pressure within the valve chamber nor the requirement that while the backseat is thus in positive engagement, the design be such as to have the capability of replacing the stem packing so that the valve can again be operated in normal fashion after having been subjected to a high temperature such as a fire. In certain environments, these operational requirements are particularly desirable and may be insisted upon by the user.